Contemporary image scanners typically are configured to produce relative motion between an array of light sensitive elements mounted in a scanning head and the image that is intended to have its contents converted to output signals manageable by data processing. That is, some such scanners sometimes move the image relative to the array while others move the array relative to the image and still others move both the image and the array. Generally, such scanners employ drive motors for moving a carriage that contains the scanning head with the motor intended to cause the scanning to occur at a relatively constant speed. Some systems will employ scanner sampling that is physically controlled by the position of the scanning head so that the sample is relatively independent of the scanner movement thereby producing a relatively constant image scanning rate or pixel density against the image under consideration.
Some prior art systems recognize that the drive motor for the scanner carriage may have variations in speed and attempt to control the sampling rate so that it is correlated to the particular scanning motor speed at a given time. One example of this is shown in U.S. Pat. No. 4,591,727 by Gaebelein et al.
In yet another form of scanner, the potential overloading of the output buffer by failure of the receiving position or workstation or the like is shown in U.S. Pat. No. 4,478,514 by Bell. In this case, the speed of the drive motor for the scanner is actually correlated directly to the amount of remaining storage space in the output buffer for the scanner. Thus, if the relative pixel density is to have uniformity across the image scanning, it is also necessary to correlate the scanner sampling rate in such an apparatus with the particular velocity of the scanner motor at any given moment.
None of the known prior art devices allow the correlation of the speed of scanning in direct relation to a predetermined pixel density so that constant scanner head sampling is possible.